Cardiac output is the measure of the amount of blood that heart pumps each minute. Current standard for clinical cardiac output measurement is bolus thermodilution method. In the method cooled (or room temperature) indicator solution is injected into the right heart and blood temperature is measured downstream in the pulmonary artery. Cardiac output can be calculated using the obtained blood temperature curve.
With bolus thermodilution method cardiac output information is obtained only at the measurement instants. However, other hemodynamic parameters, such as blood pressure, heart rate, and blood oxygenation can be monitored continuously. Continuous measurement of cardiac output would complete the real-time picture of the hemodynamic status of the patient.
Since 1960's, there have been numerous attempts to replace indicator injection by heating the blood with a resistive heater attached to the catheter, but the accuracy and reliability of the measurement has been rather poor. During recent years, continuous thermodilution methods have developed to a level, where reliable and accurate clinical use of the method is possible and at present stand-alone monitoring devices are commercially available.
However, when it comes to the question of integrating continuous cardiac output (CCO) measurement into a modular multiparameter patient monitor, there are several points to take into account. In CCO measurement accurately regulated power source for blood heating and an accurate measurement of blood temperature at the pulmonary artery are needed.
In U.S. Pat Nos. 5,594,375 and 5,636,638 is disclosed a power source for CCO measurement. The problem of the disclosed power source is, however, that it has more than one power stages and lots of components. These make it inefficient to use and difficult to install to a small cases in the measurement systems.
The real challenges in the integration are related to the heating power source. Heating of the catheter filament requires approximately 12-15 W output power. In order to reduce the risk of ventricular fibrillation in fault conditions, the heating power is to be fed to the catheter in form of 100 kHz sine wave. Because a typical measurement module of a modular patient monitor consumes less than 2 W power, the thermal properties of the module cases are designed to handle only this amount of power dissipation. Therefore, the fundamental problem at present is to develop a heating circuit, which produces the heating signal with high efficiency. In addition, the limited space in the module sets additional constraints for electronics design.
Thus, there is an increasing need for a heating circuit that drives heating energy to the catheter filament and fits to a given small space: In one example the dimensions of the CCO heater module are 107 mm×87 mm×38 mm. The size of the module sets limits to the circuit area and used components, but even more importantly it sets limits for the heat that may be dissipated into the module case.
The objective of the present invention is to provide a circuit that meets the above-mentioned requirements.
Another objective of the present invention is to provide a heater circuit that has as high efficiency as possible, which saves from a lot of problems and costs in thermal issues.
Another objective of the present invention is to provide a heater circuit, which has as small amount of components as possible and which can be used also in connection with small size modules as mentioned above.
As for the features characteristic of the invention, reference is made to them in the claims.